ENH: Allow crawler_main to operate on a list of paths

Summary

For https://gitlab.indiscale.com/caosdb/customers/umg/management/-/issues/235. Allow a list of directories for crawler_main and scan_directory so that we can loop over them without creating individual status records and mail notifications.

Focus

Essentially uses the fact that the underlying scan_structure_element function already supports a list of StructureElements.

Test Environment

New integration/system test should be sufficient.

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